Future regulations on emissions require significantly reduced production of pollutants from an internal combustion engine. Engine manufacturers have responded by developing a number of methods and systems directed at reducing pollutants. Exhaust gas recirculation (EGR) is a scheme to reduce the formation of NOx, an uncertain mixture of oxides of nitrogen. In an exhaust gas recirculation system, exhaust gas is introduced into the inlet air stream. The exhaust gas replaces a portion of oxygen that would normally occupy a combustion chamber. Reducing oxygen in the combustion chamber slows the combustion process. Slowing the combustion process reduces a peak gas temperature in the combustion chamber. High peak gas temperatures in the combustion chamber are generally associated with the formation of NOx.
Many current exhaust gas control schemes attempt to control a mass of oxygen available for combustion. These systems typically focus on manners of measuring mass flow in the inlet stream and mass flow of recirculated gas. These systems further use control valves to vary the mass flow of recirculated gas using a pressure differential between the inlet air stream and the exhaust gas. Relying on the pressure differential limits the effectiveness of these systems. As inlet air stream pressures increase with increasing loads, less exhaust gas recirculates into the inlet stream. To achieve greatly reduced emissions envisioned by future regulations, exhaust gas must be recirculated over a wider operating range, especially in the high load high speed range where NOx production increases rapidly.
In U.S. Pat. No. 4,426,848 issued to Stachowicz on Jan. 24, 1984, a venturi nozzle expands the operating range of the exhaust gas recirculation system. The venturi nozzle creates a reduced pressure area in the inlet stream where the exhaust gas may be introduced. However, these systems reduce the effectiveness of a pressure charging device and ultimately reduce engine performance. In U.S. Pat. No. 5,517,976 issued to Bachle on May 21, 1996, a single piston or group of pistons forces exhaust gas into the inlet stream wherein the pistons act as positive displacement pumps. This system provides a fixed volumetric percentage of recirculated exhaust gas. In a low load condition at low speeds, the air to fuel ratio may be so low that using the fixed volumetric percentage of exhaust gas may prevent proper combustion. The present invention is directed at overcoming one or more of the problems set forth above.